Light olefins, such as ethylene, serve as feeds for the production of numerous chemicals. Olefins traditionally are produced by petroleum cracking. Because of the limited supply and/or the high cost of petroleum sources, the cost of producing olefins from petroleum sources has increased steadily.
Alternative feedstocks for the production of light olefins are oxygenates, such as alcohols, particularly methanol, dimethyl ether, and ethanol. Alcohols may be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastics, municipal wastes, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for olefin production.
The total yield slate for a typical oxygenate to olefin process includes (a) light saturates and oxygenates, i.e. methane, hydrogen, carbon monoxide, carbon dioxide, and ethane, and (b) heavier by-products with a molecular weight higher than propylene, i.e. C.sub.4 's and C.sub.5 's. A typical oxygenate to olefin process has a methane selectivity of no less than about 5 molar % or 2.5 wt %.
The literature related to oxygenate to olefin processes focuses on maximizing ethylene and propylene product yields. Little attention has been given to optimizing the total yield slate. One reason for this lack of attention may be that the light saturate by-products have no real fouling potential and also have some value--at least as fuel. However, it is costly to separate the light saturate by-products from the desired olefin products.
Various modifications have been made to molecular sieve catalysts having intermediate sized pores to increase the selectivity of these intermediate pore catalysts to olefins. However, little attention has been given to treatments to increase the selectivity of small pore catalysts to olefins.
Small pore zeolitic catalysts have a tendency to deactivate rapidly during the conversion of oxygenates to olefins. A need exists for methods to decrease the rate of deactivation of small pore zeolitic catalysts during such conversions.
Small pore silicoaluminophosphate (SAPO) molecular sieve catalysts have excellent selectivity in oxygenate to olefin reactions. However, a continuing need exists for treatments which will maximize the production of olefins and minimize the production of light saturate byproducts using small pore molecular sieve catalysts, generally, in order to reduce the cost of such processes and render them commercially viable.